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MELSEC-Q
10 TROUBLESHOOTING
10.1.4 Reading the data communication status (Transmission sequence status)
This section explains how to read the current status of the data communication using
the MC protocol stored in the buffer memory.
When GX Configurator-SC is used, check the status using the "MC protocol monitor"
screen (see Section 8.6.4).
When a transmission problem occurs, read operation is executed to check the status
of the data communication using the MC protocol.
(1) Transmission sequence status storage area (address: 255
H
and 265
H
)
The status of data communication using the MC protocol is stored as a numerical
value in buffer memory addresses 255
H
and 265
H
.
b15
to
b0
0 to 9
0 to 9
Buffer memory address
265
H
(Information on the CH1 side)
(Information on the CH2 side)
255
H
The following shows the correspondence between the numerical values in the
transmission sequence status storage area and the data communication status.
"6" "1"..."6" is repeated afterward.
Values 7 to 9 are stored when "mode switching" or "initialization
of transmission sequence" is performed.
E
N
Q
Station
number HL
Request
to CPU
Response
from
CPU
ACK
or
NAK
Message wait
Request from an external device
External
device
Q series C24
Stored
v
alue
103456
2
REMARK
When the target interface is not set to use the MC protocol, "0" is stored in the
transmission sequence status storage area.
(2) Example of a program that reads from transmission sequence
status storage area
The following shows an example of a program that reads from the transmission
sequence status storage area.
(input/output signals X/Y00 to X/Y1F of the Q series C24)
Example of FROM instruction
b15
to
b0
0
4
D0
D1
The CH1 side is waiting to receive a command message,
and is using non procedure or bidirectional protocol.
CH2 is accessing the PLC CPU upon reception of a
command message.
Reads the status of data communication
on the CH1 side from address 255H.
Reads the status of data communication
on the CH2 side from address 265H.
Read command